1. Field of the Invention
The present invention concerns a filter circuit which is used for optical and/or magnetic recording devices such as communications equipment, audio machines, or HDD·MO, etc.
2. Description of the Related Art
Filter circuits are used in communications equipment or audio machines for selecting a specified frequency and removing noise. A low pass filter, for example, transmits low-band frequency signals. A high pass filter transmits high-band frequency signals. And a band pass filter transmits frequency signals within a specified frequency range.
In recent years, electronic appliances must meet high precision demands, and filter circuits used in electronic circuits mounted on these electronic appliances are no exception. These filter circuits must also function effectively over a wider frequency range. The wide frequency range requirement, in particular, has become more demanding for electronic appliances and their filter circuits. Thus, a filter circuit which operates over a wide frequency range while maintaining high precision has become necessary.
FIG. 1 shows a trans-conductance filter circuit (gm filter circuit) used for a trans-conductance amplification circuit (gm amplification circuit). The gm filter circuit is formed by a first transistor 1 and a second transistor, which receive differential input signals. Three constant current sources 3, 4, and 5 are connected to the first transistor 1 and the second transistor 2. A pair of capacitors 6 and 7 are connected to differential output terminal.
The cut-off frequency fc of the gm filter circuit is expressed by formula 1:fc=gm/(2πC)  (1)
where gm=Δi/ΔV, and
where gm=Mutual conductance; C=Capacitance; Δi=The variation in current magnitude; ΔV=The variation in voltage magnitude.
In the context of realizing the wide frequency range objective in an electronic appliance, it is necessary to vary the cut-off frequency of its gm filter circuit. The following two methods are used for varying the cut-off frequency. First, the current provided to the gm filter circuit may be varied. Second a variable resistor may be inserted in the gm circuit between the transistors as shown in FIG. 4.
First, the current provided to the gm filter circuit may be varied. FIG. 2 shows varying the current values of the constant current sources of the filter circuit. FIG. 3 shows the relationship between the drain current (Vd) and mutual conductance (gm) of a MOS transistor. In this figure, gm bears an attribute proportional to the square root of Vd. gm increases as the current value (Vd) increases. As shown by formula (1), the cut-off frequency also increases as gm increases. As the current value (Vd) decreases, both gm and the cut-off frequency decreases.
Second, a variable resistance may be inserted into the gm filter circuit. FIG. 4 shows inserting a variable resistance between the respective sources of a first transistor 15 and a second transistor 16, which receive the differential input signal for the filter circuit. A variation of gm is induced by varying the resistance, and the cut-off frequency is accordingly varied. As the resistance value increases, the variation in current magnitude Δi decreases. When the variation in current magnitude decreases, gm also decreases. As a result the cut-off frequency is lowered. As the resistance value decreases, the magnitude of the current variation (Δi) increases. As a result, gm and the cut-off frequency increase.
These methods for varying cut-off frequencies, however, have problems. For example, a constant current source may be formed from a transistor used within a constant current region. An effort to vary the cut-off frequency, however, may result in the transistor operating outside this region.
A general relationship between the source-drain voltage (Vds) and source-drain current (Ids) of a transistor is shown in FIG. 5. In this figure, the tripolar tube region I (resistant region) signifies a region over which an Ids variation dovetails a Vds variation. The pentapolar tube region II (constant current region) signifies a region over which virtually no Ids variation is incurred despite a Vds variation. The constant current source is predicated on the use of the pentapolar tube region II (constant current region) of the transistor.
When the cut-off frequency variation is small, only a small current variation is required. This operating condition is shown by the range of curves a, b, and c in FIG. 5. The existence of a pentapolar tube region can be found and constant current feature can be used for the transistor.
When the cut-off frequency is raised, a larger current value is required. This operating condition is shown by curve d in FIG. 5. No pentapolar tube region or only a narrow pentapolar tube region exists at this operating condition. Thus, at this operating condition, the constant current feature cannot be used for the transistor. Consequently, it is impossible to provide normal actions of the gm circuit requiring a constant current source. From the standpoint of securing the normal action of the gm filter circuit (i.e., securing the constant current attribute for the transistor), the magnitude of the current variation is limited. Consequently, the range of the cut-off frequency variation is also limited.
The current mirror circuit shown in FIG. 6 is one example of a constant current source. FIG. 7 illustrates a second example of a constant current source. The transistors (27) and (29) are vertically stacked for improving the constant current feature.
FIG. 8 illustrates a gm filter circuit that uses a current mirror circuit as a constant current source. In this mirror circuit, the transistors (35), (37), (39), (42), and (44) are stacked vertically in relation to the gm filter circuit of FIG. 1. These vertically stacked transistors improve the ability of the circuit to provide a constant current. They are problematic, however, in that they reduce the output signal amplitude. This amplitude reduction is a grave problem where minimizing the electric power consumptions of electronic circuits is a priority.
When a large range in the cut-off frequency is required, it is necessary to induce a significant variation of the resistance value of the variable resistance. When the variable resistance value is large or becomes large, however, the differential circuit function becomes lost, and the circuit performance becomes equivalent to a circuit having two inputs and one output. It also becomes impossible to exploit the advantages of the differential circuit such as its advanced power source variation and noise resistance. Thus, the resistance value cannot be adjusted to a large value from the standpoint of preserving the differential circuit performance, and the range for the cut-off frequency is limited.
Thus, the second method, wherein a variable resistance is inserted into the gm filter circuit places limits on the range of the cut-off frequency. Similarly, the first method, wherein the current through the gm filter circuit is varied, also places limits on the range of the cut-off frequency. The amplitude reduction of the output voltage when using vertically stacked transistors exists as another problem.